Factors influencing the feeding habits of the ghost crab Ocypode quadrata (Fabricius, 1787) on subtropical sandy beaches

Abstract

Studies on feeding habits are fundamental to characterising the trophic role that species play in an ecosystem. They also help to understand the dynamics of populations involved in ecological relationships. Among the existing populations on tropical and subtropical sandy beaches in the world, the ghost crab (Ocypode) is the most conspicuous invertebrate. Despite its wide distribution, there is arguable information on the trophic ecology of this crab. Our study thus aimed to characterize the diet of the Ocypode quadrata on three morphologically distinct beaches in southern Brazil. Considering such factors as environmental differences, seasons, sex, and ontogeny, we evaluated which of these would be responsible for differences in the crab's food composition and quantity of items consumed. In addition, we used the repletion index (RI) to check the presence of food resource throughout the year. The samples were collected monthly, from March 2017 to February 2018, on three subtropical beaches in southern Brazil. We performed the biometry of the crabs, which involved weighing and measuring the size of the individuals, followed by the weighing of the stomachs to calculate the repletion index. We examined the contents of 660 stomachs, which were grouped into major food categories. Feeding index (IAi) indicates that feeding patterns varied only between seasons, demonstrating that the availability of prey is reflected in the diet of the ghost crab. The most significant items in its diet were insects, unidentified organic matter, and crustaceans. The RI tended to increase as the sand temperature increased. An increase in daytime foraging activity during cold months in contrast to a greater night-time foraging during warmer months shows a plasticity in the time of foraging. Finally, we conclude that the variety of prey consumed suggests that the O. quadrata has mainly opportunistic detritivorous feeding habits, as well as other feeding modes.